Parkinson's misdiagnosed
New condition explains some movement disorders and dementia.
28 January 2004
HELEN PEARSON
Some patients diagnosed with Parkinson's disease or the wear and tear of
ageing may actually be suffering from a recently discovered brain disease,
doctors say.
The disease, called FXTAS, strikes around 1 in 3,000 men over the age of 50,
according to Paul Hagerman of the University of California, Davis, and his
colleagues. It causes gradually worsening symptoms of tremors, difficulty in
initiating movement and memory loss.
Hagerman suspects that as many as 10% of patients diagnosed with atypical
Parkinson's, in which patients tremble when they move, might actually have
FXTAS. He hopes that doctors will now screen these patients for FXTAS with a
simple genetic test.
Larger studies must be done before doctors can be sure about how common
FXTAS really is, cautions Stephen Warren, who studies the disorder at Emory
University in Atlanta, Georgia. But, he says, "I'd buy the argument that
many of these patients are diagnosed with something else."
Common people
Researchers have known about FXTAS for a few years. It is caused by a
mutation in a brain-growth gene called FMR1, which sits on the X chromosome.
One section of the gene repeats itself over and over again, like a stutter.
In people with 200 of these repeats, the FMR1 gene is switched off. They
develop fragile-X syndrome, the most common form of inherited mental
retardation, which affects roughly 1 in 3,600 boys, and affects girls to a
milder extent.
A further 1 in 800 people carry between 55 and 200 genetic repeats, called a
premutation. Once thought to be harmless, researchers have since discovered
that the premutation boosts the activity of the FMR1 gene which kills off
nerve cells, leading to FXTAS.
Hagerman assessed 99 people who carry premutations, and found that around
one-third of those over the age of 50 showed signs of FXTAS. Women may
escape the disease, probably because they have another X chromosome to
compensate for the defective one.
Mixed bag
Doctors suspect that Parkinson's and other coordination disorders, called
ataxias, are actually ragbags of diseases lumped together because of similar
symptoms. They sometimes unearth genes that can explain a handful of such
cases. FXTAS seems to be relatively widespread for a disease caused by a
single gene.
There are few data yet on whether the prognosis for FXTAS is better than
that for Parkinson's, says team member Elizabeth Berry-Kravis of Rush
University in Chicago. But a correct diagnosis might prevent the
prescription of inappropriate drugs; some patients in the study had even had
unnecessary brain surgery thanks to the misdiagnosis.
As researchers understand what causes FXTAS, they can also start working on
drugs tailored to treat it, Berry-Kravis says. They have a head start, she
adds, because a lot is already known about the gen
References
1. Jacquemont, S. et al. Penetrance of the fragile-X associated
tremoe/ataxia syndrome in a premutation carrier population. Journal of the
American Medical Association, 291, 460 - 469, (2004). |Article|
<http://jama.ama-assn.org/cgi/content/abstract/291/4/460>
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